1. Field of the Invention
The present invention relates to a magnetoresistance element having magnetization stability and showing high magnetoresistance ratio. The present invention relates also to a magnetic memory device (nonvolatile memory device) employing the magnetoresistance element.
2. Related Background Art
The magnetoresistance effect type memory device conducts recording by magnetization direction of a magnetic layer corresponding to digital information. This type of memory device does not require energy supply from the outside for memory retention, and can be produced by a simple process in comparison with a semiconductor memory device without limitation of the substrate material. Therefore, this type of memory device is promising as an inexpensive nonvolatile memory device having a large capacity.
FIG. 1 is a schematic sectional view of an example of constitution of a conventional magnetoresistance effect type memory device. This magnetoresistance effect type memory device as shown in FIG. 1 has basically a sandwich structure having nonmagnetic layer 52 between two ferromagnetic layers 51, 53. The process for detecting the recorded information is classified roughly into two types. The function of the ferromagnetic layers 51, 53 differs depending on the type of the detection process.
A first type of the process is described below.
In the first type of process, two ferromagnetic layers 51, 53 are constituted to be different in the coercivity: the layer of lower coercivity serving as a detection layer, and the other layer of higher coercivity serving as a memory layer. The coercivity of ferromagnetic layers 51, 53 is differentiated usually by changing the constituting chemical elements or composition of the layers or by changing the layer thickness.
The recording is conducted by application of a recording magnetic field (Hw) greater than the coercivity of the memory layer to parallelize the magnetization direction of the memory layer to the recording magnetic field (Hw). This process is explained by reference to FIGS. 2A and 2B. FIGS. 2A and 2B are schematic drawings illustrating the states of the recorded information in a conventional magnetoresistance effect type memory device. In FIGS. 2A and 2B, the device comprises memory layer 61 having higher coercivity, nonmagnetic layer 62, and detection layer 63 having ferromagnetism of lower coercivity than memory layer 61.
In FIG. 2A, for example, the symbol "0" denotes the state that memory layer 61 under nonmagnetic layer 62 is magnetized in a - direction (leftward direction), and as shown in FIG. 2B, the symbol "1" denotes the state that memory layer 61 is magnetized in a + direction (rightward direction). Immediately after the recording, the magnetization directions of the both layers are parallel, since the coercivity of detection layer 63 is less than that of memory layer 61.
The detection is conducted by applying direct electric current at a prescribed intensity to the memory device, applying simultaneously thereto a magnetic field (Ha) less intense than the coercivity of memory layer 61, and measuring the potential change caused by reversal of magnetization in detection layer 63. In the state of the parallel magnetization of both magnetic layers, the resistivity of the element is lower than that in the state of the antiparallel magnetization.
The states are shown in FIGS. 3A to 3C through FIG. 6. FIGS. 3A to 3C and FIGS. 5A to 5C are schematic drawings for explaining the change of magnetization of the respective layers by application of a magnetic field. In these drawings, the same symbols as in FIGS. 2A and 2B are used for the corresponding members without special explanation. FIGS. 4 and 6 are respectively a timing chart showing the potential change by application of a magnetic field: the abscissa showing the time t, and the ordinate showing the electric potential V.
As shown in FIGS. 3A to 3C, on application of a detecting magnetic field of +Ha to a memory device having a record of the state "0", for example, as shown in FIG. 3A, the magnetization directions of the both magnetic layers (memory layer 61, and detection layer 63) become antiparallel as shown in FIG. 3B to raise the electric potential, and on subsequent application of the magnetic field of -Ha, the magnetization directions become parallel to lower the potential. FIG. 4 shows the change of the electric potential.
Similarly, as shown in FIGS. 5A to 5C, on application of detecting magnetic field of +Ha to a memory device having a record of state "1", as shown in FIG. 5A, the magnetization directions of the both magnetic layers become parallel as shown in FIG. 5B to lower the electric potential, and on subsequent application of the magnetic field of -Ha, the magnetization directions become antiparallel to raise the electric potential. FIG. 6 shows the change of the electric potential. In this type of the detection, detected signal depends only on the magnetization direction of the memory layer independently of the magnetization direction of the detection layer before the detection, which enables precise detection of the information recorded in the memory layer.
Another type of process is described by reference to FIGS. 7A and 7B. FIGS. 7A and 7B show schematically magnetization directions of magnetoresistance effect type memory device. In this process, as shown in FIGS. 7A and 7B, two ferromagnetic layers holds a nonmagnetic layer 72. One of the two ferromagnetic layers is employed as fixed-magnetization layer 71 which is magnetized in a fixed direction, and the other one is employed as memory layer 73. The magnetization of memory layer 73 is forced to be parallel to the direction of the applied magnetic field. The fixed-magnetization layer 71 can be formed by giving coercivity greater than the recording magnetic field to the ferromagnetic layer. The magnetization-reversing field in the fixed magnetization direction can be intensified by formation of exchange-coupling with an antiferromagnetic layer.
FIG. 8 shows shift of the magnetization loop of a film of "ferromagnetic layer/antiferromagnetic layer" formed by exchange-coupling of a ferromagnetic layer and an antiferromagnetic layer. FIG. 8 shows the rightward shift corresponding to Hex (exchange-coupling magnetic field). As shown in FIG. 8, the magnetization-reversing field is Hex.+-.Hc (where Hc is the coercivity of the ferromagnetic layer). Therefore, the stronger magnetic field is required for reversing the magnetization by application of a magnetic field in a direction of the shift of the magnetization loop than that for a ferromagnetic single layer.
In the case where the shift of the magnetization loop is greater than the coercivity, namely Hex&gt;Hc, the magnetization is directed, under zero magnetic field, invariably to a fixed direction. In such a case, even if the magnetization is reversed by some cause, the magnetization returns to the original state, requiring no initialization.
With such a magnetoresistance effect type memory device, information is detected by applying a magnetic field, to the memory device in a state of absence of application of a magnetic field, at a magnetic field intensity for reversing the magnetization of memory layer 73 in a prescribed direction, and measuring the change of the output voltage. This process is shown in FIGS. 10A and 10B through FIG. 13. FIGS. 10A and 10B and FIGS. 12A and 12B illustrate schematically the change of magnetization of the respective layers on application of a magnetic field. In these drawings, the same symbols as in FIGS. 7A and 7B are used for denoting the corresponding members without explanation. FIGS. 11, 12A, 12B and 13 are respectively a timing chart for illustrating the change of the electric potential on application of a magnetic field with the abscissa representing time t, and the ordinate representing an electric potential V.
In FIGS. 10A and 10B and FIGS. 12A and 12B, the fixed magnetization is directed rightward, and the magnetization of memory layer 73 is directed rightward in state "0" and directed leftward in state "1". In FIGS. 10A and 10B, application of a detecting magnetic field does not change the magnetization direction of memory layer 73 having the record "0", causing no change in the output voltage as shown in FIG. 11. On the other hand in memory layer 73 having the record "1", as shown in FIGS. 12A and 12B, the magnetization of the two ferromagnetic layers changes from an antiparallel state to a parallel state as shown in FIGS. 13 to decrease the magnetoresistance to lower the detected voltage.
Such a detection method, however, will erase the recorded information, and will require rewriting for retaining the information. For avoiding the erasure and rewriting of the information, the information should be detected without application of a magnetic field. Therefore, the values of the output voltage of the state "0" and of the state "1" should be known preliminarily.
For obtaining a high S/N ratio, the magnetoresistance ratio is preferably higher. The magnetoresistance ratio depends on the material of the magnetic layer, the state of the interface between the layers, and other factors. In a spin-scatter type film constitution, it depends also on the thickness of the nonmagnetic layer between the magnetic layers. This phenomenon is called a shunt effect, by which an increase of thickness of the nonmagnetic layer lowers the magnetoresistance ratio owing to the increase of electrons flowing through the nonmagnetic layer without participating in the change in magnetoresistance. In view of the shunt effect, the nonmagnetic layer is preferably thinner.
The memory device utilizing the magnetoresistance change, when employed for mobile information instrument, should be capable of memorizing in high density. In such a memory device, the cell area should be made smaller. For the fine working, useful are photolithography and FIB (focused ion beam etching) employed in a semiconductor process.
As described above, in the spin-scatter type magnetoresistance effect type memory device, the nonmagnetic layer held between the ferromagnetic layers is preferably thinner in view of the shunt effect. However, the thinner nonmagnetic layer causes increase of a magnetostatic coupling field between the ferromagnetic layers. This magnetostatic coupling field serves to parallelize the magnetization direction in the two ferromagnetic layers. Accordingly, this greater magnetostatic coupling field retards the change of the magnetization into the antiparallel state, preventing the change in magnetoresistance.
The intensity of diamagnetizing field of the ferromagnetic body depends on its shape: the smaller ratio of the length in easy magnetization direction to the layer thickness increases the intensity of the diamagnetizing field. In other words, if the area of the memory cell is decreased without changing the layer thickness of the magnetic layer of a magnetoresistance effect type memory device, the diamagnetizing field is intensified to render the magnetization unstable, which makes impossible the retention and detection of information. On the other hand, the decrease of the thickness of the magnetic layer is limited in view of formation of uniform thickness of a magnetic thin film for high-density memory. The same problem arises in a spin-tunnel type memory device having a nonmagnetic layer constituted of the insulation film.
Thus, the material for the ferromagnetic layer of a high-density memory device has preferably uniaxial magnetic anisotropy and sufficiently high coercivity. However, such a material has not been found yet.